In place of pulp paper, synthetic paper, which comprises a substrate layer of a biaxially-stretched polypropylene film and, as formed on its surface and back, a paper layer of a monoaxially-stretched polypropylene film comprising from 8 to 65% by weight of a fine inorganic powder, has been proposed and put into practical use (for example, Patent References 1, 2, and 3). The synthetic paper is characterized in that, as compared with pulp paper, it has extremely high water resistance, and, for example, it is not swollen or broken by rainwater.
An application example of the synthetic paper, taking advantage of its water resistance, is paper for posters. Depending on the production purpose, they may require multi-color printing excellent in glossiness. In particular, travel posters, movie posters, campaign posters, and posters for commercial sale promotion for cars, refreshing drinks, etc., glossy multi-color offset printing is liked for producing a light and springy atmosphere.
Heretofore, such poster paper is to appeal to sight as if glossy printing be made thereon, for example, by laminating a transparent polyvinyl chloride film or the like on printed synthetic paper. The poster of the type laminated with a transparent resin film is problematic in that its production process is complicated and is disadvantageous in point of the schedule and the cost thereof.
As a method of overcoming the problem, high-glossy synthetic paper has been proposed (for example, Patent References 4 and 5) and put into practical use. The high-glossy synthetic paper of the type comprises a transparent resin layer of a propylene resin not substantially comprising a filler, as previously formed as a surface layer to be printed thereon. The high-glossy synthetic paper is printed thereon to give high-glossy printed matter. However, the transparent resin layer has a very little surface roughness capable of receiving ink penetrating thereinto. When a solvent-type ink or an oxidation drying-type ink is used for the synthetic paper of the type, then the vehicle, a component of the ink (low-molecular oil component) may penetrate into the resin of the surface layer, and therefore the ink fixation is good with no problem. However, in case where a UV-curable ink that has become popularized recently is used for printing on the synthetic paper, the ink component does not almost penetrate into the synthetic paper but would dry and solidify on the surface layer while it is just thereon; and therefore, though the ink could fix in some degree on the paper just after printing thereon but when it is exposed outdoor as posters, its fixation may lower within a relatively short period of time owing to the influence of sunlight and rainwater on it, and there occurs a problem in that the ink on the printed surface may drop off.
Specifically, the success in obtaining the conventional synthetic paper having a surface layer of a polypropylene resin (propylene homopolymer) and having excellent surface glossiness results from the omission of the surface filler so as to reduce the surface roughness as much as possible; however, the surface roughness capable of receiving ink to penetrate thereinto is extremely reduced, and therefore the fixation of UV-curable ink on the synthetic paper, especially the UV-curable ink fixation thereon under the condition based on the premise of using it outdoors is poor. Accordingly, the market has a great demand for high-glossy synthetic paper and for improving it to attain good UV-curable ink fixation thereon and to have good weather resistance capable of keeping it.    [Patent Reference 1]: JP-B 46-40794    [Patent Reference 2]: JP-A 56-141339    [Patent Reference 3]: JP-A 56-118437    [Patent Reference 4]: JP-A 61-3748    [Patent Reference 5]: JP-A 1-295845